This invention relates, in general, to monolithic operational amplifiers, and more particularly, to monolithic operational amplifiers capable of driving capacitive loads while maintaining sufficient phase and gain margins.
In general, operational amplifiers include on chip compensation schemes to improve their stability. It is well known in the art that an operational amplifier configured for closed loop operation will be stable if an excess phase shift of an open loop gain is greater than -180.degree. at a gain of unity. Further, two well known figures of merit describing amplifier stability are a phase margin and a gain margin in the open loop configuration. The phase margin is defined as a difference between the excess phase shift, at unity gain, and -180.degree.. The gain margin is defined as a difference between the value of the open loop gain, at a frequency at which the excess phase shift is -180.degree., and an open loop gain of unity.
An important method for improving operational amplifier stability is Miller compensation wherein a Miller compensation capacitor is placed in a feedback path of the operational amplifier. The compensation capacitor moves an input pole to a lower frequency creating a dominant pole and moves any output poles to higher frequencies; a technique commonly known as pole-splitting. The dominant pole ensures the open loop gain reaches unity before the excess phase shift becomes -180.degree..
In addition, Miller compensation may introduce a right half plane zero. In applications requiring low power consumption, the right half plane zero may become dominant and degrade the phase and gain margins of the operational amplifier. A resistor placed in series with the Miller compensation capacitor may move the right half plane zero such that it becomes a left half plane zero. A frequency at which the left half plane zero occurs is a function of a value of the resistor, a transconductance of an output stage transistor, and the Miller compensation capacitor. The resistor improves the phase margin and the stability of the operational amplifier. However, the zero created by the series combination of the resistor and the compensation capacitor may decrease the gain margin at higher frequencies, thus the stability of the operational amplifier may be decreased. Moreover, operational amplifiers having a compensation capacitor or a series combination of the resistor and the compensation capacitor may be unable to drive large capacitive loads. Accordingly, it would be advantageous to have a compensation scheme capable of driving a range of capacitive loads while maintaining sufficient phase and gain margins.